build-sys: rename to _execdir

build-sys: rename to _execdir

The variable name "usrlibexecdir" is very confusing (because we have
/usr/libexec). The "exec" prefix is required for user-defined
directories, see

   http://www.gnu.org/software/hello/manual/automake/The-Two-Parts-of-Install.html#The-Two-Parts-of-Install

This patch renames all usr*execdir variables to usr*_execdir.

Signed-off-by: Karel Zak <kzak@redhat.com>